[WPT] Fix images-are-ignored for Firefox.
Firefox draws the broken image icon for the non-loaded image,
so make the test use alt text to prevent the broken image icon.
Change-Id: Ic0c882109e760d208e6795b93350c1d4d7cc939b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5805897
Reviewed-by: Philip Rogers pdr@chromium.org
Commit-Queue: Stephen Chenney schenney@chromium.org
Auto-Submit: Stephen Chenney schenney@chromium.org
Cr-Commit-Position: refs/heads/main@{#1346367}